1. Field of the Invention
This invention relates generally to subsea oilfield tubing operations and systems, and more particularly to operations and systems in which tubing is used for subsea wellbores in marine and offshore drilling and wellbore locations.
2. Background of the Art
Oilfield wellbores are drilled by rotating a drill bit conveyed into the wellbore by a drill string. The drill string includes a drilling assembly (also referred to as the xe2x80x9cbottom hole assemblyxe2x80x9d or xe2x80x9cBHAxe2x80x9d) and tubing that carries the drill bit. The tubing may be coiled tubing or jointed pipe. The drilling assembly usually includes a drilling motor or xe2x80x9cmud motorxe2x80x9d that rotates the drill bit and a variety of sensors for taking measurements of a variety of drilling, formation and BHA parameters. A suitable drilling fluid (commonly referred to as the xe2x80x9cmudxe2x80x9d) is supplied or pumped under pressure from the surface down the tubing. The drilling fluid drives the mud motor and discharges at the bottom of the drill bit. The drilling fluid returns uphole via the annulus between the drill string and the wellbore and is returned to the surface work station via a return line.
For drilling wellbores under water (referred to in the industry as xe2x80x9coffshorexe2x80x9d or xe2x80x9csubseaxe2x80x9d drilling), a supply of tubing is carried at the surface work station (for example, located on a vessel or platform). A rig, which may have one or more tubing injectors, is used to move the tubing into and out of (trip) the wellbore. U.S. Pat. No. 08/911,787, assigned to the assignee of this application, provides certain methods of injecting tubing into subsea wellbore, which is incorporated herein by reference as if fully set forth herein. A riser, which is formed by joining sections of casings or pipes, maybe deployed between the surface work station and the wellhead equipment. The riser is utilized to guide the tubing toward the wellhead. The riser also serves as a conduit for the fluid returning from the wellhead to the sea surface. The riser is substantially larger in diameter than the wellbore and is designed so as not to leak the drilling fluid into the surrounding water. To deploy the riser, sections of pipe (usually 30-40 feet long) are serially connected at the drilling platform and deployed under water. Such large diameter jointed pipes or tubing are very heavy and thus impose significant loads on the surface work station and in particular the rigs and injectors used to deploy the riser.
One suitable injector for deploying the riser is shown in U.S. Pat. No. 5,850,874, commonly assigned to the applicant. While the high speed operation of such injectors can be useful in reducing the time for deployment of the tubular riser, holding the upper reach of a long string of riser against slippage in the injector and against human error in the operation of the injector can be a problem. Once the injector loses its hold on the riser, it is free to fall to the sea bed, with resultant damage to the riser and other subsea equipment. Similar problems can arise with drill strings or other tubing strings in subsea operations in deep water and/or in deep wellbores. Such drill strings are thus also long and heavy, so that they too must be securely held in the injector. Failure to do so will result in the drill string dropping into the wellbore, which may be difficult or perhaps impossible to retrieve.
In an alternative design to the above-noted tubular riser for conveying the return fluid from the subsea wellhead to the surface work station, the return line may be separate and spaced apart from the drill sting tubing. Such return lines are typically smaller and lighter than the jointed pipe/tubing riser, and indeed may be constructed of a flexible, non-metallic material. However, such construction results in the return line leaving the drill string tubing unprotected from the elements of the subsea environment. Indeed, the return line may actually come to interfere with the movement of the tubing toward and away from the subsea wellbore, if the surface work station is a ship or other moveable platform that allows the return line and the tubing to become twisted or wrapped together, upon angular movement of the platform. It is known that the water currents near the sea surface can cause great turbulence in the drilling equipment that extends from the drilling vessel to the wellbore. It is also known that sea water corrodes the drilling equipment that extends from the drilling vessel to the wellbore.
A riser that extends the full distance from the surface to the wellhead to hold drill fluid protects the drilling equipment extending from the vessel to the wellbore both mechanically, such as from upper level turbulence, and chemically, such as from corrosion. Applicants, however, have found that such turbulence is relatively minor past 150-200 feet from the sea surface and that corrosion is also relatively small after such depths.
The methods and apparatus of this invention overcome many of these tubing handling problems encountered in subsea tubing handling operations. For the problem of securely holding the upper reach of heavy tubular strings suspended from the surface work station, whether the string be the riser, the drill string or any other oilfield work string or whether it is a string of coiled tubing or jointed pipe, this invention provides an automatic safety device to prevent the loss of such string. This safety device supplements the rig or injector, by providing an automatic stop at the surface work station to grip and hold the string if the rig or injector does not. Indeed, such safety device is even useable at on-shore and shallow water drilling sites having shorter lengths of string and thus are at less risk of lost pipe for mechanical (if not operator error) reasons.
The present invention further provides for the reduction in the overall weight of the drill string and/or work string formed from continuous or coiled tubing suspended from the surface work station toward a work site in a wellbore. Such string has a first length or segment of coiled tubing shorter than the total length needed to reach from the surface work station to the wellbore work site, and second or upper length of coiled tubing to make up the difference having characteristics different from the first or lower length of coiled tubing. A tubular connector is provided to secure the lengths together so as to preserve the overall mechanical and pressure integrity of the string. Thus, the string can be designed to have a lighter and more flexible lower segment and a stronger (and perhaps larger) upper segment. Other differences in characteristics as between the length of tubing are also contemplated.
Similarly, the invention enables the use of a separate and distinct return line (rather than a riser) without the problems of leaving the drill string unprotected and avoiding the tendency of the return line and the drill string to wrap together. For the latter problem, the work moveable platform is provided with a turntable or other moveable device for passage of both the drill/work string and the return line thereto at spaced apart locations and then holding the string and return line in a predetermined spaced relationship. This reduces the tendency of these members to twist about each other.
The present invention further eliminates the need for the complete, full length riser. In the present invention a relatively short (about 200 feet) large diameter tubing (referred to herein as an xe2x80x9cisolation tubexe2x80x9d) may be deployed below the drilling surface work platform to negate the impact of turbulence and the corrosive effect of the sea water near the sea surface. The isolation tubing may be formed of a lighter gage material than a conventional riser and is filled with a suitable non-corrosive, non-water soluble fluid whose fluid density is less than that of the sea water. Such a fluid remains within the isolation tubing. A separate return line carries the return fluid from the wellhead to the surface work station.
Examples of the more important features of the invention have been summarized rather broadly so that in order that the detailed description thereof that follows may be better understood, and so that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.